Fluorinated hydrocarbons are widely used in solvent degreasing, foam blowing, and the construction of refrigeration and air conditioning equipment, for example. Many industrial processes produce waste airstreams containing low concentrations of fluorinated hydrocarbons including chlorofluorocarbons (CFCs) and halons (fluorinated hydrocarbons containing one or more bromine atoms). In the manufacture of these chemicals, and in industrial plants using them, streams with relatively high concentrations of halocarbons are also encountered. Table I shows that enormous amounts of the two most common fluorine-containing halocarbon solvents, CFC-11 (CCl.sub.3 F) and CFC-12 (CCl.sub.2 F.sub.2) are emitted into the atmosphere each year. It is estimated that only 10% of the total CFC-11 and CFC-12 produced is recycled. The remaining 90% is released into the atmosphere.
TABLE 1 ______________________________________ Use and Worldwide Emission of Freons Regulated by the Montreal Protocol Estimated 1980 Emissions Chemical Major Use (1,000 tons) ______________________________________ CFC-11 Rigid and flexible polyurethane foam, 266 refrigeration, and air-conditioning CFC-12 Rigid and flexible polyurethane foam, 378 aerosol, sterilization, refrigeration, air conditioning, and food freezing CFC-113 Degreasing and cleaning agent 80 in electronics manufacturing industry CFC-114 Rigid nonpolyurethane foam -- CFC-115 Refrigeration, air conditioning -- ______________________________________
Fluorinated hydrocarbons are expensive, making their recovery from effluent streams and their subsequent reuse very economically attractive. The current failure of producers and users to recover more than small amounts of these compounds is therefore indicative of the inadequacy of present treatment and recovery methods. It is imperative that CFCs and similar compounds be removed from effluent streams because of scientific evidence linking them to depletion of the ozone layer. The United States and many other nations have signed an agreement entitled the "Montreal Protocol on Substances that Deplete the Ozone Layer". The Montreal Protocol calls for progressive production freezes on the following chemicals: CFC-11 (CCl.sub.3 F), CFC-12 (CCl.sub.2 F.sub.2), CFC-113 C.sub.2 Cl.sub.3 F.sub.3), CFC-114 (C.sub.2 Cl.sub.2 F.sub.4), CFC-115 (C.sub.2 ClF.sub.5), Halon-1211 (CF.sub.2 ClBr), Halon-1301 (CF.sub.3 Br) and Halon-2402 (C.sub.2 F.sub.4 Br.sub.2). It is planned to reduce the production of CFCs to 50% of the 1986 levels in the next decade and to limit the production of halons to the 1986 levels. However, conventional CFCs will probably continue to be manufactured until the year 2000. An efficient method of reducing emissions of these environmentally harmful solvents is urgently needed. Even when the present levels of emissions are reduced, and/or alternative, environmentally safer substitutes, such as the HCFCs, are found, there will be a continuing need for recovery systems both from an economic and a pollution point of view.
Conventional systems for treating airstreams contaminated with organic solvent vapors involve incineration, carbon adsorption and compression condensation. Fluorinated hydrocarbons are difficult to treat by condensation processes because of their volatility. They are generally not appropriate for treatment by incineration because they are non-flammable. Carbon adsorption can only be used efficiently for very dilute streams, because the operating and capital costs of the plants increase with increasing solvent concentration in the feed. Also, high concentration leads to unacceptably high temperatures in the carbon bed because of the exothermic adsorption step. Consequently process streams must frequently be diluted many-fold before being passed to the carbon beds. Regeneration of the beds makes labor and maintenance requirements onerous and expensive. In addition, some fluorinated solvents are not stable during the steam regeneration cycle, leading to corrosion of the system, and some low boiling compounds are not adequately adsorbed. Compression condensation is only suitable for highly concentrated streams and for solvent streams than can be brought to their dew point without the need for excessive cooling and/or the application of very high pressures. A number of fluorinated hydrocarbons have very low boiling points and are unsuitable for treatment by compression condensation. Efficient economic separation processes, capable of handling streams in the range from those containing a few ppms of fluorinated hydrocarbons to those where the fluorinated hydrocarbon forms the major component of the stream, would therefore represent a major advance in the field.
That membranes have the potential to separate organic vapors from air is known in the art. For example, U.S. Pat. No. 4,553,983, commonly owned with the present invention, describes a process for separating airstreams containing low concentrations of organic vapor (2% or less) from air, using highly organicselective membranes. U.S. Pat. No. 3,903,694 to Aine describes a concentration driven membrane process for recycling unburnt hydrocarbons in an engine exhaust. U.S. Pat. No. 2,617,493 to Jones describes separation of nitrogen from concentrated hydrocarbon feedstreams.
The permeability of a gas or vapor through a membrane is a product of the diffusion coefficient, D, and the Henry's law sorption coefficient, k. D is a measure of the permeant's mobility in the polymer; k is a measure of the permeant's sorption into the polymer, and depends in part on the condensability of the vapor. The diffusion coefficient tends to decrease as the molecular size of the permeate increases, because large molecules interact with more segments of the polymer chains and are thus less mobile. On this basis alone, one would expect that organic vapors, which in general are large molecules compared with oxygen and nitrogen, would have much lower permeabilities than air through most polymers. With rigid, glassy polymer materials this is generally true. In elastomeric membrane materials however, the effect of the sorption coefficient can be dominant. Particularly for easily condensable, reactive organic compounds, the sorption coefficient in a rubbery material may be so high that the material exhibits a high or very high organic permeability. The ideal selectivity, defined as the ratio of the permeabilities measured with pure gas or vapor streams, for the organic over air may then also be very high. For example, organic/nitrogen ideal selectivities up to 1,000 or more have been measured in some rubbery polymers for acetone, trichloroethane, toluene and octane. Of course, it is recognized that the actual selectivity obtained in a real process may be substantially lower than the ideal selectivity, because the organic concentration in the mixture is low, or because the organic component swells the membrane, thereby increasing the permeation of nitrogen and oxygen. Fluorinated hydrocarbons as a class are relatively inert, and exhibit high volatility and hence poor condensability. Thus, from their general properties it might be predicted that the selectivities of rubbery polymers for fluorinated hydrocarbons would be relatively modest, compared with the figures above. In fact, this prediction is borne out experimentally. A product bulletin from General Electric gives the ideal selectivities of CFCs 11, 12, 22, 114 and 115 over air as between 50 and 2. A paper by Roberts and Ching, of SRI, entitled "Recovery of Freon Gases with Silicone Rubber Membranes" discusses the permeabilities of certain fluorinated hydrocarbons through silicone rubber, and reports many results where the fluorinated hydrocarbon was less permeable than air. Thus, theoretical predictions, coupled with the available teachings in the art, would suggest that fluorinated hydrocarbons are relatively poorly separated from air by membranes, compared with other organic vapors, and are not good candidates for treatment by means of a membrane-based system. To applicants' knowledge, membrane processes that can treat fluorinated hydrocarbon-contaminated streams, leaving a residue stream form which the majority of the hydrocarbon has been removed, and/or producing a permeate from which the hydrocarbon can be recovered for reuse, have not previously been available to the art.